1. Field of the Invention
The present disclosure relates to an image reading apparatus having light sources which lights in respective colors.
2. Description of the Related Art
A light emitting diode (hereinafter referred to as LED), which emits lights of red (R), green (G) and blue (B) colors, is used as a document illumination light source of an image reading apparatus of a copying machine and a multifunction printer. A document reading unit includes a contact image sensor (hereinafter referred to as CIS), which uses the LED as a light source.
In the CIS, which uses a three-color RGB LEDs as a light source, a color filter is not applied to a line sensor which receives diffused light from a document. When performing color reading of a document with the CIS, the R, G, and B LEDs are turned on in order and in synchronization with a line cycle, which is a reading cycle of the line sensor, to perform RGB color separation of document image.
On the other hand, when using the three-color RGB LEDs as a light source for document illumination, following problems occur. A first problem is that “color misregistration” occurs in a sub scanning direction of read image. To form a single line color image, each of the R, G, and B colors is read in order by one third lines. This causes the color misregistration in the sub scanning direction occurs. Each of the RGB color may be shifted at the document illumination position in the sub scanning direction, or at the image reading position, which causes the color misregistration. This theoretically produces, for example, the color misregistration of one third lines in the sub scanning direction.
A second problem is that there needs to adjust incident light quantity of diffused light from a document to the line sensor. Generally, the LED has large variation in emitted light quantity. Therefore, to avoid saturation of input and output levels to the line sensor, signal input range to an analog front end (hereinafter referred to as AFE), which is connected to a line sensor output post stage, needs to be set within an appropriate range. Generally, the signal input range is set by increasing or reducing LED turn-on period (turn-on time) to adjust the incident light quantity to the line sensor within an appropriate range. On the other hand, when adjusting the turn-on period in such a manner, LED turn-off period occurs within the line cycle of the line sensor. Therefore, if there is any fine line extending in the sub scanning direction of the document, sufficient illumination cannot be applied to the fine line. This result in a deterioration in reading accuracy of the fine line.
An image reading apparatus is disclosed in U.S. Pat. No. 6,545,777, which is a proposal to the first problem as mentioned above. The image reading apparatus decreases color misregistration amount in the sub scanning direction by controlling LED turn-on timing of each of the R, G, and B LEDs which turns on within a line cycle of a line sensor. Specifically, to optimize an output level of the line sensor, the image reading apparatus adjusts the LED turn-on period. Thereafter, assuming that the R and B LEDs are respectively turned on before and after the G LED is turned on, for example, the LED turn-on periods of the R and B LEDs are respectively controlled with respect to the LED turn-on period of the G LED. The LED turn-on periods of the R and B LEDs are adjusted in a time-series manner such that the respective LED turn-on periods of the R and B LEDs will conform to the cycle of the turn-on period of G LED (U.S. Pat. No. 6,545,777, FIG. 9). It is noted that the line cycle of the line sensor and the turn-on timings of the each of the R, G, and B LEDs within the cycle affect the color misregistration amount in the sub scanning direction. Therefore, the above adjustment is employed. Through the above adjustment, the turn-on period of R LED gets closer to that of G LED. Similarly, through the control, the turn-on period of G LED gets closer to that of B LED. As a result, when the document is irradiated with the light, the position misregistration among the RGB colors is reduced, thus the color misregistration is reduced.
An image reading apparatus disclosed in U.S. Pat. No. 7,884,978 attempts to provide a proposal to the second problem as mentioned above. The image reading apparatus comprises LED turn-on control circuit, by which turn-on timing of each of the R, G, and B LEDs can individually be changed with CIS having two LEDs of the illumination light source. In the image reading apparatus, in one LED turn-on timing adjustment, a turn-on start position is fixed near a line cycle start position of a line sensor. By changing a turn-on end position, the turn-on period is adjusted. In the other LED turn-on timing adjustment, the turn-on end position is fixed near a line cycle end position of the line sensor. By changing the turn-on start position, the turn-on period is adjusted. This reduces a period during which the LED is turned-off within the line cycle.
According to the description of U.S. Pat. No. 6,545,777, color misregistration amounts in the sub scanning direction can be reduced. On the other hand, an image processing affected by the color misregistration includes, for example, processing of determining whether character on a read document is black character or not. Also, the image processing includes processing of automatically determining whether a read document is color document or monochrome document. In this case, deterioration of determination accuracy cannot be prevented only by the reduction of the color misregistration, which is a problem.
Also, to perform determination processing as mentioned, it is necessary to perform color misregistration correction processing to solve the color misregistration to generate an image in which color misregistration is almost solved. However, there is an individual variability in the incident light quantity of LED. Therefore, when the LED turn-on time is adjusted for every LED or every image reading apparatus, the turn-on period of each of the R, G, and B LEDs differs depending on the apparatus. U.S. Pat. No. 6,545,777, FIG. 9 illustrates LED turn-on period of each of the R, G, and B LEDs. If a period from turn-on end of R LED to turn-on start of G LED and a period from turn-on end of G LED to turn-on start of B LED differ in each apparatus, the color misregistration amount produced on an image in the sub scanning direction accordingly differs in each apparatus. Therefore, the color misregistration amount in the sub scanning direction differs in each apparatus depending on the apparatus.
According to the description of U.S. Pat. No. 7,884,978, the LED turn-off period within the line cycle of the line sensor can be reduced. However, when the LED turn-on time is adjusted for every image reading apparatus, the turn-on period of each of the R, G, and B LEDs differs depending on the apparatus. A center position of the turn-on period of each of the R, G, and B LEDs within the line cycle also changes for every apparatus. Similar to the image reading apparatus as disclosed in U.S. Pat. No. 6,545,777, the period from turn-on end of R LED to turn-on start of G LED differs from the period from turn-on end of G LED to turn-on start of B LED at a center of the LED turn-on period. Therefore, the color misregistration amount produced on the image in the sub scanning direction differs accordingly. As a result, the color misregistration amount in the sub scanning direction differs depending on the apparatus.
Accordingly, to perform the color misregistration correction processing when LED turn-on control, described in U.S. Pat. No. 6,545,777 and U.S. Pat. No. 7,884,978, is performed, it is necessary to measure the color misregistration amount for every apparatus and perform the color correction processing based on the measured result (for example, parameter setting). As a result, the increased number of steps, including measuring the color misregistration amount in a production process, incurs cost increase of product.